I ticketed a simple AONE4 ex-JNB (FWIW dealing by phone entirely with the "AA" office in Cape Town) and I'm pretty sure I got AA's YR pricing on the JNB-xLHR-LAX code-share that starts my ONE I used ITA multi-city to see the taxes and was within $50 of the amount I paid (and the difference between AA's YR and BA's YQ is more like $200 iirc).

I also believe that you can't use an AA code-share JNB-LHR or vv. It has to be to/from a point beyond LHR, e.g. JFK.

BA had an I-fare LHR-JNB-LHR for something around USD2500 that i pasted around the RTW. I think I've seen fares on that route under $2000 in the last year.